U.S. patent application number 10/588113 was filed with the patent office on 2007-05-17 for optical head and optical disk device.
Invention is credited to Hideki Aikoh, Osamu Mizuno, Takeharu Yamamoto.
Application Number | 20070109923 10/588113 |
Document ID | / |
Family ID | 35782804 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070109923 |
Kind Code |
A1 |
Mizuno; Osamu ; et
al. |
May 17, 2007 |
Optical head and optical disk device
Abstract
[Problem]To provide an optical head having a satisfactory shock
resistance and capable of holding an aberration correction lens
without consuming power and achieving accurate positioning. [Means
for Resolution] An aberration correction lens 4 is disposed in a
space between a laser light source 3 and an objective lens 5, and a
lens holder 10 is frictionally coupled to a drive shaft 7 via a
frictional holding body 8. A piezoelectric element 6 is provided to
one end of the drive shaft 7. The piezoelectric element 6 extends
and contracts in response to an applied voltage. The lens holder 10
is moved relatively with respect to the drive shaft 7 in the drive
shaft direction by varying a change rate when the applied voltage
to the piezoelectric element 6 is increased and decreased.
Inventors: |
Mizuno; Osamu; (Osaka,
JP) ; Aikoh; Hideki; (Osaka, JP) ; Yamamoto;
Takeharu; (Osaka, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK L.L.P.
2033 K. STREET, NW
SUITE 800
WASHINGTON
DC
20006
US
|
Family ID: |
35782804 |
Appl. No.: |
10/588113 |
Filed: |
June 30, 2005 |
PCT Filed: |
June 30, 2005 |
PCT NO: |
PCT/JP05/12091 |
371 Date: |
July 31, 2006 |
Current U.S.
Class: |
369/43 ;
G9B/7.086; G9B/7.095; G9B/7.131 |
Current CPC
Class: |
G11B 7/0937 20130101;
G11B 7/0946 20130101; G11B 7/13927 20130101; G11B 7/1376 20130101;
G11B 7/0948 20130101 |
Class at
Publication: |
369/043 |
International
Class: |
G11B 21/10 20060101
G11B021/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 2004 |
JP |
2004-199135 |
Claims
1-18. (canceled)
19. An optical head comprising: a laser light source emitting a
flux of light; an objective lens, the flux of light from the laser
light source to an optical disk passing through the objective lens;
a lens holder holding an aberration correction lens in a space
between the laser light source and the objective lens; a drive
shaft disposed to extend in a direction parallel to an optical axis
of the flux of light, the drive shaft guiding the lens holder in
the extended direction; a piezoelectric element provided at an end
portion of the drive shaft, the piezoelectric element being capable
to extend and contract in a drive shaft direction in response to an
applied voltage; and a position detection portion detecting a
position of the aberration correction lens in the drive shaft
direction, wherein the lens holder is movable relatively with
respect to the drive shaft in the drive shaft direction by varying
a change rate when the applied voltage to the piezoelectric element
is increased and decreased.
20. The optical head according to claim 19, wherein: a voltage that
gives a rise to a change causing the drive shaft to slide with
respect to the lens holder and a voltage that gives a rise to a
change causing the drive shaft to move integrally with the lens
holder are repetitively applied to the piezoelectric element.
21. The optical head according to claim 19, wherein: the position
detection portion includes a magnetic field generation portion and
a magnetic field detection portion disposed to be allowed to
undergo displacement with respect to the magnetic field generation
portion in the optical axis direction.
22. The optical head according to claim 21, wherein: the drive
shaft is supported on a base having a bottom portion, the magnetic
field detection portion disposed so as to protrude from the bottom
portion of the base.
23. The optical head according to claim 21, wherein: more than one
magnetic field detection portion is disposed in a line along the
drive shaft direction.
24. The optical head according to claim 21, further comprising: a
correction magnetic field detection portion provided at a position
unsusceptible to a magnetic field generated by the magnetic field
generation portion in such a manner that a direction of magnetic
field sensitivity is aligned with a direction of magnetic field
sensitivity of the magnetic field detection portion.
25. The optical head according to claim 24, wherein: a correction
magnetic field generation portion is provided adjacently to the
correction magnetic field detection portion.
26. The optical head according to claim 21, further comprising: an
auxiliary guiding shaft made of a soft magnetic body, the auxiliary
guiding shaft disposed parallel to the drive shaft, wherein the
magnetic field generation portion is disposed at a position at
which a direction heading from the magnetic field generation
portion to the auxiliary guiding shaft becomes perpendicular to the
drive shaft.
27. The optical head according to claim 19, wherein: the lens
holder comes in contact with the drive shaft via a frictional
holding body.
28. The optical head according to claim 19, wherein: the drive
shaft is disposed parallel to the optical disk; the optical head
further comprises an auxiliary guiding shaft disposed parallel to
the drive shaft and a mirror that deflects a flux of light from the
laser light source in a direction of a normal to the optical disk;
and the mirror is disposed in a space between the aberration
correction lens and the objective lens and also in a space between
the drive shaft and the auxiliary guiding shaft.
29. The optical head according to claim 27, wherein: the frictional
holding body is made of a resin material containing a
fluorine-based compound or fluorine-based resin.
30. The optical head according to claim 19, further comprising: a
temperature sensor that detects a temperature of the optical
head.
31. The optical head according to claim 19, wherein: the aberration
correction lens corrects spherical aberration.
32. An optical disk device comprising: the optical head according
to claim 19; and a control portion that adjusts an applied voltage
to the piezoelectric element according to a detection result on the
optical head by the position detection portion.
33. The optical disk device according to claim 32, wherein: the
control portion is configured to be capable of acquiring disk
identification information provided to the optical disk; and the
optical disk device further comprises a storage portion that stores
a set position of the aberration correction lens in response to the
disk identification information.
34. An optical disk device comprising: the optical head according
to claim 24; and a control portion that adjusts an applied voltage
to the piezoelectric element according to a detection result on the
optical head by the position detection portion, wherein the control
portion is configured to correct a set position of the aberration
correction lens according to a detection result on the optical head
by the correction magnetic field detection portion.
35. The optical disk device according to claim 34, wherein: the
optical head includes a correction magnetic field generation
portion provided adjacently to the correction magnetic field
detection portion.
36. An optical disk device comprising: the optical head according
to claim 30; and a control portion that adjusts an applied voltage
to the piezoelectric element according to a detection result on the
optical head by the position detection portion, wherein the control
portion is configured to correct a set position of the aberration
correction lens according to a temperature detected by the
temperature sensor.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical head having a
system for correcting spherical aberration of a light spot
irradiated onto an optical disk, an optical disk drive, and an
optical disk device.
BACKGROUND ART
[0002] In order to meet an ever-increasing data recording density
of an optical disk in recent years, the wavelength of a laser beam
is becoming shorter and the NA of the objective lens is becoming
higher for an optical head used to record and play back data. An
optical head using the objective lens having a high NA, however,
has a problem that it is quite sensitive to influences of spherical
aberration caused by an error of thickness of the cover layer of
the optical disk used as a recording medium.
[0003] To solve this problem, as is disclosed in Patent Document 1,
an optical head provided with spherical aberration correcting means
has been known. The optical head disclosed in Patent Document 1
performs electromagnetic driving by supporting a lens used to
correct the spherical aberration on ATTACHMENT B leaf springs.
[0004] Hereinafter, the configuration of the optical head will be
described with reference to FIG. 20. As is shown in FIG. 20, the X
axis is used for the optical axis direction. An aberration
correction lens 41 is mounted on a lens holder 44, and a coil 42 is
wound around the lens holder 44. A magnetic field is applied to the
coil 42 by a magnet 43.
[0005] Leaf springs 45 are connected to an aberration correction
base 46. Each blade spring 45 supports the lens holder 44 to be
movable chiefly in the direction X. By providing two leaf springs
45, the aberration correction lens 41 is allowed to move in
parallel easily in the X axis direction. Also, by making the blade
plate 45 to be of a folding structure, it is possible to suppress a
displacement of the aberration correction lens 41 in the Y axis
direction caused by bending of the leaf springs 45. A position
sensor 47 that detects the position of the aberration correction
lens 41 in the optical axis direction is provided. In this example,
the position sensor comprises an optical sensor.
[0006] When a specific DC current is supplied to the coil 42, the
lens holder 44 receives thrust in the optical direction due to the
function of the magnetic field induced by the magnet 43, and the
leaf springs 45 bend accordingly. The aberration correction lens 41
then undergoes relative displacement with respect to the aberration
correction base 46. In this instance, the aberration correction
lens 41 stops and stands still at a position at which an elastic
restoring force of the leaf springs 45 and the thrust that the coil
42 is receiving achieve equilibrium. The position sensor 47
generates a signal corresponding to the position of the aberration
correction lens 41 in this instance, which enables position control
to correct a position error from the target position by performing
feedback control on a current value of the coil 42 as needed.
[0007] A flux of light having passed through the aberration
correction lens 41 changes its divergent-convergence state with the
position in the optical axis direction (the direction X), which
gives rise to spherical aberration. Spherical aberration caused in
this instance is the aberration inverse to the spherical aberration
caused by a thickness error of the cover layer of the optical disk
when the flux of light goes incident on the objective lens. The
spherical aberration of a light spot irradiated onto the optical
disk can be therefore corrected by the aberration correction lens
41.
[0008] Patent Document 1: Japanese Patent No. 3505525 (pp. 4-6,
FIG. 4)
DISCLOSURE OF THE INVENTION
[0009] The optical head in the related art described as above,
however, has problems as follows.
[0010] That is to say, the aberration correction lens 41 is moved
in the X axis direction; however, because the aberration correction
lens 41 is supported on the leaf springs 45, not only it oscillates
in the X axis direction, but it also rotates about the Y axis to no
small extent. Hence, when the system is subjected to a disturbance
and starts to oscillate about the Y axis, it is no longer
observable or controllable. The same applies to the displacement
about the Z axis and the displacement in the Y axis direction, that
is, in the buckling direction of the leaf springs.
[0011] As a result, even when the aberration correction lens 41
oscillates in a direction other than the X axis direction, it fails
to suppress a flux of light coming out from the aberration
correction lens 41, which gives rises to a recording error,
defective playback, etc. of the optical disk.
[0012] In addition, because it is necessary to keep feeding a
current to the coil 42 to let the aberration correction lens 41
stand still so as not to undergo displacement, power consumption is
increased.
[0013] Further, when a multi-layer optical disk is used as the
subject in order to increase the density, it is necessary to widen
a movable range of the aberration correction lens 41. With the
configuration in the example of the related art in which the
aberration correction lens 41 is supported on the leaf springs 45,
however, a lens movement in the Y axis direction is no longer
negligible when the movable range is widened. Furthermore, elastic
strain energy of the leaf springs 45 is increased as the aberration
correction lens 41 undergoes significant displacement. This raises
a problem that holding power increases correspondingly. In short,
there is a problem that the related art is substantially
inadaptable to a multi-layer optical disk.
[0014] An object of the invention is therefore to provide an
optical head being capable of holding the aberration correction
lens without consuming power and having a satisfactory shock
resistance and achieving accurate positioning.
[0015] In order to achieve the above and other objects, an optical
head of the invention is an optical head that irradiates a flux of
light from a laser light source onto an optical disk through an
objective lens, including: a lens holder that holds an aberration
correction lens in a space between the laser light source and the
objective lens; a driving shaft that is disposed to extend in a
direction parallel to an optical axis of the flux of light and
guides the lens holder in the extended direction; a piezoelectric
element that is provided at an end portion of the driving shaft and
extends and contracts in a driving shaft direction in response to
an applied voltage; and a position detection portion that detects a
position of the aberration correction lens in the driving shaft
direction, wherein it is configured in such a manner that the lens
holder is moved relatively with respect to the driving shaft in the
driving shaft direction by varying a change rate when the applied
voltage to the piezoelectric element is increased and
decreased.
[0016] In this optical head, when the driving shaft is oscillated
in the axial direction by applying a voltage to the piezoelectric
element, a displacement rate differs when the driving shaft
undergoes displacement in one direction and when the driving shaft
undergoes displacement in the other direction. Hence, when the
displacement takes place at a high rate, sliding occurs between the
driving shaft and the lens holder, while no sliding occurs between
these two components when the displacement takes place at a low
rate. It is thus possible to move the aberration correction lens
gradually in the optical axis direction as the position of the lens
holder with respect to the driving shaft changes gradually while
the driving shaft repetitively oscillates. The aberration
correction lens can be therefore positioned accurately in the
optical axis direction. Moreover, it is sufficient to apply a
voltage to the piezoelectric element only when the aberration
correction lens is displaced, and no power is necessary when the
aberration correction lens is allowed to stand still. Further,
because the lens holder is supported on the driving shaft, it is
possible to control the aberration correction lens not to undergo
displacement in a direction other than the optical axis
direction.
[0017] As has been described, according to the invention, the
aberration correction lens can be fixed at any position on the
driving shaft without consuming any power; moreover, accurate
positioning is enabled. Further, the shock resistance can be
enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a view schematically showing a major portion of an
optical head according to a first embodiment of the invention.
[0019] FIG. 2 is a side view of the optical head.
[0020] FIG. 3 is a characteristic view schematically showing a
relation of a frictional force generated between a driving shaft
and a frictional holding body provided in the optical head and a
relative velocity of the driving shaft.
[0021] FIG. 4 is a view showing a magnet provided in the optical
head.
[0022] FIG. 5 is a view used to describe respective cross sections
of the magnet.
[0023] FIG. 6A is a view showing a magnetic flux in a cross section
Y1; FIG. 6B is a view showing a magnetic flux in a cross section
Y2; and FIG. 6C is a view showing a magnetic flux in a cross
section Y3.
[0024] FIG. 7 is a characteristic view showing a relation of the
position of an aberration correction lens in the optical axis
direction and a position signal.
[0025] FIG. 8 is a view schematically showing a major portion of an
optical disk device according to a second embodiment of the
invention.
[0026] FIG. 9 is a view schematically showing a major portion of an
optical disk device according to a third embodiment of the
invention.
[0027] FIG. 10 is a characteristic view showing a temperature
change of the position signal.
[0028] FIG. 11 is a view schematically showing a major portion of
an optical head according to a fourth embodiment of the
invention.
[0029] FIG. 12 is a characteristic view showing a change of the
position signal of the optical head caused by the position of the
aberration correction lens in the optical axis direction.
[0030] FIG. 13 is a view schematically showing a major portion of
an optical disk device according to a fifth embodiment of the
invention.
[0031] FIG. 14 is a view schematically showing a major portion of
an optical disk device according to a sixth embodiment of the
invention.
[0032] FIG. 15 is a view schematically showing a major portion of
an optical head according to a seventh embodiment of the
invention.
[0033] FIG. 16 is a side view of the optical head.
[0034] FIG. 17 is a view schematically showing a major portion of
an optical head according to an eighth embodiment of the
invention.
[0035] FIG. 18 is a schematic plan view of the optical head.
[0036] FIG. 19A is a view showing a magnet and a hall element in a
ninth embodiment of the invention; FIG. 19B is a view showing
another configuration; and FIG. 19C is a view showing still another
configuration.
[0037] FIG. 20 is a perspective view showing a major portion of an
optical head in the related art.
BEST MODE FOR CARRYING OUT THE INVENTION
[0038] Hereinafter, the best mode for carrying out the invention
will be described in detail with reference to the drawings.
First Embodiment
[0039] FIG. 1 and FIG. 2 are views schematically showing a major
portion of a first embodiment of an optical head of the
invention.
[0040] As is shown in FIG. 1 and FIG. 2, the optical head includes
a laser light source 3, an aberration correction lens 4, and an
objective lens 5. A laser beam 3a emitted from the laser light
source 3 is irradiated onto an optical disk 1 used as a recording
medium by passing through the aberration correction lens 4 and the
objective lens 5. The optical disk 1 has at least a substrate 2, a
cover layer 2a, and a recording layer (not shown) disposed between
the substrate 2 and the cover layer 2a. The recording layer may be
made of a phase change material, a magneto-optical material, or any
other recording material.
[0041] The aberration correction lens 4 is supported on an
aberration correction base 11. To be more concrete, the aberration
correction lens 11 includes a bottom portion 11a, a pair of first
supporting portions 11b provided to stand on the bottom portion
11a, and a pair of second supporting portions 11c provided to stand
on the bottom portion 11a. The bottom portion 11a is made into a
flat plate in the shape of a capital T when viewed in a plane. The
both first supporting portions 11b are disposed at one end (for
example, the left end in FIG. 1) in a direction orthogonal to the
optical axis of the laser beam 3a, and the both second supporting
portions 11c are disposed at the other end (for example, the right
end in FIG. 1) in the direction orthogonal to the optical axis.
[0042] The both first supporting portions 11b are provided to stand
on the bottom portion 11a while being spaced apart in the optical
direction. Each first supporting portion 11b is made into a shape
of a flat plate disposed parallel to a direction orthogonal to the
optical axis of the laser beam 3a. A fixing portion 11d is provided
to stand on the bottom portion 11a of the aberration correction
base 11 on the side opposing one of the first supporting portions
11b with the other first supporting portion 11b (the lower one in
FIG. 1) in between. The fixing portion 11d is made into a shape of
a flat plate disposed parallel to the first supporting portions
11b.
[0043] A piezoelectric element 6 is fixed to the fixing portion
11d. The piezoelectric element 6 is provided in such a manner that
when a voltage is applied, it extends slightly in a direction A
which is the driving direction in the drawing.
[0044] Each first supporting portion 11b is provided with an
through hole for a drive shaft 7. These through holes are provided
at positions to be parallel to the optical axis of the laser beam
3a. The drive shaft 7 inserted through these through holes is
therefore disposed parallel to the optical axis.
[0045] One end of the drive shaft 7 protrudes from one of the first
supporting portions 11b. The drive shaft 7 is formed in a
cylindrical shape. The drive shaft 7, being supported on the both
first supporting portions 11b, is held spaced-apart from the
aberration correction base 11, and is thereby free to move in
directions A and B shown in the drawing.
[0046] The second supporting portions 11c are fixed to the
aberration correction base 11 while being spaced apart in the
optical axis direction. Each second supporting portion 11c is made
into a shape of a flat plate disposed almost parallel to a
direction orthogonal to the optical axis of the laser beam 3a.
[0047] An auxiliary guiding shaft 9 is supported on the both second
supporting portions 11c. The auxiliary guiding shaft 9 is disposed
almost parallel to the optical axis of the laser beam 3a and at the
same time it is held by the second supporting portions 11c so as
not to move in the axial direction. The aberration correction lens
4 is positioned in a space between the auxiliary guiding shaft 9
and the drive shaft 7.
[0048] One end of the drive shaft 7 protruding from one of the
first supporting portions 11b is fixed to the piezoelectric element
6. That is to say, the piezoelectric element 6 is used as means for
providing acceleration to the drive shaft 7 to move in a direction
parallel to the optical axis of the laser beam 3a.
[0049] A lens holder 10 is supported on the drive shaft 7 and the
auxiliary guiding shaft 9. The aberration correction lens 4 is
fixed to the lens holder 10. The lens holder 10 is made into a
shape of a rectangular flat plate. An insert groove 10b is provided
in the end portion of the lens holder 10 on the drive shaft 7 side,
and a guiding groove 10a is provided in the end portion of the lens
holder 10 on the auxiliary guiding shaft 9 side.
[0050] A cylindrical frictional holding body 8 is inserted
immovably into the insert groove 10b in the lens holder 10. The
drive shaft 7 is inserted through the frictional holding body 8.
The frictional holding body 8 has a sufficient length to prevent
the aberration correction lens 4 from tilting.
[0051] The frictional holding body 8 and the drive shaft 7 are
frictionally coupled to each other. In other words, when an
external force that is gradually increasing at or lower than the
static friction force acts on the drive shaft 7 the frictional
holding body 8 moves integrally with the drive shaft 7, since a
friction force to some extent is exerted between the frictional
holding body 8 and the drive shaft 7. Meanwhile, when the external
force acting on the drive shaft 7 increases abruptly to the extent
that the inertia force corresponding to the mass of a movable
portion 100 described below exceeds the static friction force,
sliding occurs between these two components, which causes the drive
shaft 7 alone to move. For example, as is shown in FIG. 3, the
static friction force f1 is exerted when a relative velocity
between the drive shaft 7 and the frictional holding body 8 is
lower than a specific rate v1. When the inertia force exceeds the
static friction force f1, the relative velocity shifts to a dynamic
frictional range at or higher than v1, and sliding occurs between
these two components. A dynamic friction force f2 that is smaller
than the static friction force f1 is thus exerted. Hence, by
adjusting a manner in which a voltage is applied to the
piezoelectric element 6 appropriately in response to a friction
force exerted between the drive shaft 7 and the frictional holding
body 8 and the mass of the movable portion 100, it is possible to
make a switch between a sliding state in which the friction holding
body 8 (lens holder 10) undergoes relative displacement with
respect to the drive shaft 7 and an integral moving state in which
the drive shaft 7 and the frictional holding body 8 (lens holder
10) move as one unit. By repeating the both states, it is possible
to change the positional relation of the frictional holding body 8
(lens holder 10) with respect to the drive shaft 7.
[0052] It should be noted that there is no problem when the
frictional holding member 8 is formed integrally with the lens
holder 10.
[0053] The auxiliary guiding shaft 9 is inserted into the guiding
groove 10a. The guiding groove 10a and the guiding shaft 9 come
into contact with each other in a state where a friction force,
which is sufficiently small in comparison with a friction force
exerted between the frictional holding member 8 and the drive shaft
7, is exerted.
[0054] In the example shown in the drawing, the guiding groove 10a
is formed by notching the end portion of the lens holder 10.
Alternatively, a guiding hole comprising a through hole may be
provided in the lens holder 10, so that the auxiliary guiding shaft
9 is inserted through the guiding hole.
[0055] The optical head is provided with a position detection
portion 20 that detects the position of the aberration correction
lens 4 in the optical axis direction. The position detection
portion 20 includes a magnet 12 as an example of a magnetic field
generation portion and a hall element 13 as an example of a
magnetic field detection portion. The magnet 12 is provided to the
lens holder 10. Meanwhile, the hall element 13 is provided to the
bottom portion 11a of the aberration correction base 11 to face the
magnet 12. As is shown in FIG. 2, the hall element 13 is provided
to slightly protrude from the top surface (inner surface) of the
bottom portion 11a.
[0056] As is shown in FIG. 4, the magnet 12 is formed in a shape of
a rectangular prism, and comprises two wedge-shaped regions 12a and
12b partitioned at the boundary extending in a diagonal direction
with respect to the length direction. Each of the regions 12a and
12b is polarized to a different polarity, and the axis of easy
magnetization is set in a direction perpendicular to the sheet
surface.
[0057] The hall element 13 is provided to improve the sensitivity
for a magnetic field in a direction almost perpendicular to the
bottom portion 11a (a direction almost perpendicular to the sheet
surface of FIG. 1). Herein, assume that it is set so that a
positive output is obtained when the hall element 13 is subjected
to a downward magnetic field. In other words, assume that it is set
so that a positive output is obtained when subjected to a magnetic
field in a direction heading from the magnet 12 to the hall element
13 in FIG. 2.
[0058] Because the lens holder 10, the aberration correction lens 4
fixed to the lens holder 10, the frictional holding body 8, and the
magnet 12 are all allowed to slide along the drive shaft 7 in a
direction parallel to the optical axis, the lens holder 10, the
aberration correction lens 4, the frictional holding body 8, and
the magnet 12 are collectively defined as the movable portion 100
herein. In the optical axis direction, a direction to approximate
to the optical disk 1 is referred to as the direction A, and a
direction to move away from the disk 1 is referred to as the
direction B.
[0059] Because the lens holder 10 is supported on the two shafts,
the drive shaft 7 and the auxiliary guiding shaft 9, which are
parallel to each other, it is allowed to move in the optical axis
direction without any oscillation in a direction rotating about the
shafts.
[0060] Herein, a unit comprising a combination of the movable
portion 100 and a driving portion that moves the movable portion
100 is defined as an aberration correction unit 101. The driving
portion means a combination of the aberration correction base 11,
the piezoelectric element 6, the drive shaft 7, the auxiliary guide
shaft 9, and the hall element 13.
[0061] Hereinafter, operations of the optical head of the first
embodiment configured as described above will be described.
[0062] A laser beam 3a emitted from the laser light source 3 passes
through the aberration correction lens 4, and then forms an image
on the recording layer by passing through the objective lens 5 and
the cover layer 2a. In a case where the optical disk 1 causes
side-runout or decentering in this instance, the object lens 5
moves two-dimensionally, and the position control is performed to
follow such a movement.
[0063] During this operation, when a voltage is gradually applied
to the piezoelectric element 6, the piezoelectric element 6 extends
in a direction heading to A shown in FIG. 1. The drive shaft 7 thus
starts to move gradually in the direction heading to A, and the
frictional holding body 8 frictionally coupled to the drive shaft 7
also starts to move integrally with the drive shaft 7 in the
direction heading to A. In this instance, because a friction force
between the auxiliary guiding shaft 9 and the guiding groove 10a is
sufficiently small, the movable portion 100 including the
frictional holding body 8 moves gradually in the direction heading
to A. The aberration correction lens 4 consequently moves in the
direction heading to A while keeping its posture (integral moving
state).
[0064] When a voltage that has been kept applied to the
piezoelectric element 6 is abruptly stopped in this state, the
piezoelectric element 6 contracts abruptly. The drive shaft 7 thus
starts to move abruptly in a direction heading to B and returns to
its original position. In this instance, a force that accelerates
the movable portion 100 in the direction B is exerted. However, an
inertia force corresponding to its mass also acts on the movable
portion 100. Meanwhile, although the frictional holding body 8 and
the drive shaft 7 are frictionally coupled to each other, when the
inertial force exceeds the static friction force, sliding occurs
between the drive shaft 7 and the frictional holding body 8. The
relative velocity between these two components is therefore
increased and it shifts to a dynamic frictional range having a
relatively small friction force. As a result, the movable portion
100 including the aberration correction lens 4 remains at
substantially the same place (sliding state) regardless of the fact
that the drive shaft 7 is moving in the direction heading to B.
[0065] As a result of one cycle as a combination of the integral
moving state and the sliding state, the aberration correction lens
4 has moved in the direction heading to A by a distance comparable
to an extended length of the piezoelectric element 6. A quantity of
extension of the piezoelectric element 6 is minute, and so is a
quantity of movement of the aberration correction lens 4 per cycle.
Hence, by repeating the cycle until a desired quantity of movement
is achieved, it is possible to move the aberration correction lens
4 by an arbitrary quantity in the direction heading to A. This
movement is achieved by repeating the cycle at a quantity of
movement per cycle in the order of nanometer and at a high
frequency in the order of some hundreds kHz.
[0066] On the other hand, when the aberration correction lens 4 is
moved in the direction heading to B, the driving voltage to the
piezoelectric element 6 is increased abruptly and then the driving
voltage is reduced gradually. The movable portion 100 thus remains
immovable when the drive shaft 7 moves abruptly in the direction
heading to A, whereas it moves gradually when the drive shaft 7
moves in the direction heading toB. The movable portion 100
therefore moves in the direction heading to B. The aberration
correction lens 4 consequently moves in the direction heading to
B.
[0067] In a case where spherical aberration occurs due to an
irregular thickness of the cover layer 2a, the spherical aberration
can be corrected by changing an angle of incidence of a laser beam
on the objective lens 5 by moving the aberration correction lens 4
in the optical axis direction by the method described above.
[0068] The drive shaft 7 does not bend like a blade spring, and a
connection between the frictional holding body 8 and the aberration
correction lens 4 is thought to be substantially rigid. The
aberration correction lens 4 therefore will not oscillate due to
the influences of disturbance-induced oscillations as in the
example in the related art. Also, there is no need of the holding
power to allow the aberration correction lens 4 to stand still so
as not to undergo displacement as in the example in the related
art. In other words, by frictionally coupling the frictional
holding body 8 to the drive shaft 7, the lens holder 10 is able to
hold the aberration correction lens 4 in a stable manner without a
supply of power. It is thus possible to reduce power
consumption.
[0069] An actual spherical aberration correction operation is
performed by searching for the position of the aberration
correction lens 4 at which a playback signal from the optical disk
1 becomes most appropriate. The most appropriate position of the
aberration correction lens 4 differs in each disk 1 due to a
thickness error of the cover layer 2a.
[0070] In the case of a disk having two or more layers, the
position of the aberration correction lens 4 at which a signal
becomes most appropriate is searched for each layer. In the case of
a movement from layer to layer, it is advantageous in terms of
saving a time to store the most appropriate position, so that the
aberration correction lens 4 is moved to the most appropriate
position without having to perform a search again. In the
invention, a position signal necessary in this instance is obtained
from the hall element 13 that outputs a signal corresponding to a
magnetic field from the magnet 12.
[0071] The spherical aberration correction operation can be
performed while the focus servo is applied on the optical disk 1 or
the focus servo may be applied to the optical disk 1 after the
spherical aberration is corrected.
[0072] A magnetic flux that passes by the hall element 13 when the
magnet 12 having the wedge-shaped regions 12a and 12b as shown in
FIG. 4 is used will now be described with reference to FIG. 5 and
FIG. 6. The axis of easy magnetization is perpendicular to the
sheet surface. FIG. 6A conceptually shows a magnetic flux in the
magnet 12 taken along the cross section Y1 when viewed in a
direction V shown in FIG. 5. Likewise, FIG. 6B shows a magnetic
flux in the cross section Y2, and FIG. 6C shows a magnetic flux in
the cross section Y3. As is shown in FIG. 6A, the hall element 13
is chiefly subjected to an upward magnetic field within a plane
including the cross section Y1. Also, as is shown in FIG. 6B, it is
chiefly subjected to a lateral magnetic field within a plane
including the cross section Y2. In addition, as is shown in FIG.
6C, it is chiefly subjected to a downward magnetic field within a
plane including the cross section Y3.
[0073] Hence, when the aberration correction lens 4 moves in the
optical axis direction, the magnetic field to which the hall
element 13 is subjected changes continuously from an upward
magnetic flux in the cross section Y1 to the downward magnetic flux
in the cross section Y3. The position signal based on an output
from the hall element 13 thus shapes a continuous, almost straight
line as is shown in FIG. 7. This position signal is a signal after
the differential amplification or quantization.
[0074] Let P0 and P1 be the most appropriate positions of the
aberration correction lens 4 with respect to recording layers L0
and L1, respectively, in a doubly-layer disk. Let S0 and S1 be
position signals specifying the positions of the aberration
correction lens with respect to the positions P0 and P1,
respectively, of the aberration correction lens 4. The position
signal can be obtained from an output signal of the hall element
13. Herein, assume that the values S0 and S1 of the position signal
have been stored.
[0075] Assume that the aberration correction lens 4 is present at
the position P0 corresponding to the recording layer L0. When there
is a need to make an access to the recording layer L1, the stored
position signal S1 is compared with the current position signal S0.
Then, by moving the aberration correction lens 4 by repeating the
cycle described above until the position signal reaches S1, it is
possible to move the aberration correction lens 4 to the position
P1 corresponding to the recording layer L1. When returning to the
recording layer L0, the procedure is performed inversely.
[0076] In a case where the optical disk 1 has multiple recording
layers and the aberration correction lens 4 has to be moved
markedly, a relatively large movable distance can be readily
secured because the aberration correction lens 4 can be moved by a
distance as long as the drive shaft 7 in this embodiment. Also,
unlike the example in the related art, the lens offset or power
will not be increased depending on the amplitude, and this
embodiment is readily adaptable to a multi-layer optical disk.
[0077] An S/N ratio can be increased when the hall element 13 is
placed as close as possible to the magnet 12. However, when the
magnet 12 comes too close to the aberration correction base 11,
there is a risk of a collision. Hence, by taking an error into
account on the design, it is preferable that a distance to some
extent is secured between the magnet 12 and the aberration
correction base 11, and the hall element 13 alone or the hall
element 13 together with an accompanying fixing mechanism alone is
approximated to the position of the magnet 12. For example, it is
sufficient to cause the hall element 13 to slightly protrude from
the principal surface in the bottom portion 11a of the aberration
correction base 11.
[0078] For example, zinc or the like can be chosen as a material of
the frictional holding body 8, including a case where it is formed
integrally with the lens holder 10; however, resin can be used as
well. An effect of improving the abrasion resistance of the
frictional holding body 8 can be expected by using a resin material
having a self-lubricating property, such as PTFE (fluorine-based
resin). Moreover, because the need to apply a lubricant agent is
eliminated, there occurs no event that the lubricant agent flies
over onto the optical system. In addition, the frictional holding
body 8, including a case where it is formed integrally with the
lens holder 10, may be made of a resin material containing a
fluorine-based compound.
[0079] The first embodiment is of the configuration in which the
hall element 13 is disposed on the aberration correction base 11
and the magnet 12 is disposed on the movable portion 100 side.
However, they can be disposed in an opposite manner. It should be
noted, however, that the configuration to dispose the magnet 12 on
the movable portion 100 side is advantageous, because no wiring is
necessary.
[0080] The summary of the first embodiment is set forth as
follows.
[0081] (1) As has been described, a voltage that gives a rise to a
change causing the drive shaft to slide with respect to the lens
holder and a voltage that gives a rise to a change causing the
drive shaft to move integrally with the lens holder are
repetitively applied to the piezoelectric element.
[0082] (2) The position detection portion includes the magnetic
field generation portion and the magnetic field detection portion
disposed in such a manner that it is allowed to undergo
displacement in the optical direction with respect to the magnetic
field generation portion.
[0083] (3) The drive shaft is supported on the base having the
bottom portion, and the magnetic field detection portion is
disposed to protrude from the bottom portion of the base.
[0084] (4) The lens holder comes into contact with the drive shaft
via the frictional holding body.
[0085] (5) The lens holder is made of a resin material containing a
fluorine-based compound or fluorine-based resin.
[0086] (6) The aberration correction lens corrects spherical
aberration.
Second Embodiment
[0087] FIG. 8 is a view schematically showing a major portion of an
optical disk device according to a second embodiment of the
invention. An optical head 200 in this optical disk device includes
an aberration correction unit 101. The aberration correction unit
101 is the aberration correction unit described in the first
embodiment, and includes the aberration correction lens 4. In the
optical head 200, a mirror 15 is provided in a space between the
aberration correction lens 4 and the objective lens 5. The mirror
15 reflects a laser beam having passed through the aberration
correction lens 4 after it is emitted from the laser light source 3
in a direction almost parallel to the optical disk 1. The laser
beam reflected on the mirror 15 passes through the objective lens 5
with its optical axis being set in a direction almost perpendicular
to the optical disk 1, and is then irradiated onto the optical disk
1.
[0088] Herein, assume that the optical disk 1 is an information
recording medium having two recording layers. In short, it has a
recording layer L0 and a recording layer L1. Also, assume that the
optical disk 1 is provided with an identifier unique to the disk.
The configuration of the optical head 200 is basically the same as
the first embodiment except that the mirror 15 is provided.
[0089] The optical disk device includes a control portion 21 and a
storage portion 22. The control portion 21 controls a layer
switching signal 25, a position signal 23 from the optical head
200, and a drive signal 24 for the piezoelectric element 6
according to information from the storage portion 22. The position
signal 23 is the same as the position signal described in the first
embodiment. The control portion 21 extracts necessary information
from playback information 26 and store the extracted information
into the storage portion 22.
[0090] Operations of the optical disk device according to the
second embodiment configured as described above will now be
described.
[0091] A case where the identifier of the disk has not been stored
in the storage portion 22 will be described first.
[0092] When the optical disk 1 is loaded into the optical disk
device and is brought into a playback enabled state, the optical
head 200 first tries to play back the identifier of the disk, and
delivers the information thus read to the control portion 21 as the
playback information 26. The disk identifier can be read out
satisfactorily even in the presence of spherical aberration or the
like. The control portion 21 searches through the storage portion
22 for the disk identifier, and controls the storage portion 22 to
store the disk identifier when it has not been stored.
[0093] Subsequently, the control portion 21 controls the drive
signal 24 for the piezoelectric element 6 while confirming the
playback information 26, so that the aberration correction lens 4
comes to the most appropriate position for playing back the
information recorded in the recording layer L0. The aberration
correction lens 4 thus moves to the target position.
[0094] There are various methods for finding the most appropriate
position of the aberration correction lens 4 with respect to the
recording layer L0. For example, a method by which the aberration
correction lens 4 is moved gradually and a position at which the
jitter of the playback information 26 reaches the minimum is found
to the most appropriate position of the aberration correction lens
4, a method by which a position at which the amplitude of a
tracking error signal reaches the maximum in the absence of the
tracking servo is found to be the most appropriate position of the
aberration correction lens 4, etc. are possible. After the most
appropriate position of the objective lens 5 with respect to the
recording layer L0 is found by the focus control and the tracking
control, the layer is identified as the recording layer L0 by
reading out the recording layer identifier or the identifying
signal pre-recorded in the recording layer L0.
[0095] The most adequate value of the position signal 23 with
respect to the recording layer L0 is extracted as S0 in this
manner, and stored in the storage portion 22. The most adequate
value S1 of the position signal 23 with respect to the recording
layer L1 can be stored into the storage portion 22 in the same
procedure. A table of the most appropriate position signals 23 for
the aberration correction lens 4 using individual disk identifiers
as the indices can be thus created in the storage portion 22.
[0096] In a case where the identifier of the recording layer is not
used, the table of the position signals may be created by searching
for the recording layers sequentially from one end in the thickness
direction of the disk 1 used as a recording medium, and storing the
value of the position signal 23 at a position at which the
recording layer is detected in the storage portion 22 in order of
detection.
[0097] In a case where information is recorded into or information
is played back from the optical disk 1, for example, in a case
where recording and playback is performed using the recording layer
L0, an L0 layer switching command is provided to the layer
switching signal 25. The control portion 21 then takes out the most
appropriate signal S0 corresponding to the recording layer L0 from
the position signal table in the storage portion 22, and controls
the drive signal 24 of the piezoelectric element 6 while making a
comparison with the current position signal 23. The aberration
correction lens 4 is thus moved by changing the drive signal 24
until the position signal reaches almost S0.
[0098] A case where information about the disk identifier of the
optical disk 1 has been stored in the storage portion 22 will now
be described.
[0099] When the optical disk 1 is brought into a playback enabled
state, the disk identifier is played back and delivered to the
control portion 21 as playback information. The control portion 21
then reads out the position signals corresponding to the recording
layer L0 and the recording layer L1 as S0 and S1, respectively,
from the storage portion 22 according to the disk identifier.
[0100] In a case where the layer switching signal 25 includes the
L0 layer switching command, the control portion 21 extracts the
most appropriate position signal S0 corresponding to the recording
layer L0 from the storage portion 22, and controls the drive signal
24 of the piezoelectric element 6 by making a comparison with the
current position signal 23, so that the aberration correction lens
4 is moved until the position signal reaches almost S0.
[0101] When the layer switching is performed, by storing the most
appropriate signal positions S0 and S1 into the storage portion 22
in this manner, the need to perform a search more than once is
eliminated. The layer switching at a high speed is thus
enabled.
[0102] Also, by storing the disk identifier and the position signal
into the storage portion 22 in this manner, for the optical disk
that has been played back once, playback and recording of
information are enabled immediately on the basis of the information
stored in the storage portion 22 without having to perform a search
again.
[0103] There may be an optical disk having no disk identifier. In
such a case, however, S0 and S1 are searched for each time the disk
is loaded, and a problem will not occur particularly.
[0104] In a case where the optical disk 1 has three or more
recording layers, this embodiment is applicable by making an
appropriate change.
[0105] The summary of the second embodiment is set forth as
follows.
[0106] (1) As has been described, the second embodiment includes
the optical head and the control portion that adjusts an applied
voltage to the piezoelectric element according to the detection
result on the optical head by the position detection portion.
[0107] (2) The control portion is configured to be capable of
acquiring disk identification information provided to the optical
disk, and the storage portion that stores the set position of the
aberration correction lens corresponding to the disk identification
information is provided.
Third Embodiment
[0108] FIG. 9 is a view schematically showing a major portion of an
optical disk device according to a third embodiment of the
invention. This embodiment is different from the second embodiment
in that a temperature sensor 16 is provided to the optical head
201, and that an output of the temperature sensor 16 is inputted
into a control portion 28 as temperature information 27. The rest
of the configuration is the same as the second embodiment.
[0109] The characteristics of the hall element 13 and the magnet 12
described in the first embodiment vary with temperatures. For
example, as is shown in FIG. 10, even when the aberration
correction lens 4 is set at the same position, the position signal
generated on the basis of an output from the hall element 13
decreases almost linearly as the temperature increases. However,
because the temperature coefficient is almost constant, it is
possible to perform accurate control by taking this property into
account.
[0110] Operations of the optical disk device of the third
embodiment configured as described above will now be described.
[0111] Descriptions of the basic operations are omitted because
they are the same as the second embodiment. In the third
embodiment, the storage portion 22 has stored the position signals
S0 and S1 corresponding to the respective recording layers and
temperatures detected by the temperature sensor 16 at the time of
the searches for the most appropriate position as the temperature
information 27. Also, upon acquisition of the identification
information of the disk, this identification information is stored
as well.
[0112] In a case where the identification information of the disk
has not been stored in the storage portion 22, the control portion
28 records the following into the storage portion 22: the
identification information of the optical disk and the position
signals S0 and S1 at which the aberration correction lens 4 reached
the most appropriate position, plus the temperatures when the
appropriate positions were searched as the temperature information
27. The control portion 28 monitors the temperature each time the
most appropriate position is searched for, and corrects the
position signals S0 and S1 on the basis of a difference between the
temperatures at the time of the searches stored in the storage
portion 22 and the current temperature as well as the temperature
coefficient.
[0113] This correction can be made in the same manner as a
correction of the temperature coefficient for resistance. For
example, in a case where the position signal S0 is searched for
under the condition of the temperature T1, when the position signal
S0 at the temperature T2 needs to be calculated, then an equation
as follows can be used: S0(T2)=S0(T1).times.(1+.alpha.(T2-T1))
where .alpha. is a temperature coefficient that has an almost
constant value. This value can be readily found empirically.
[0114] In a case where the disk identification information has been
stored in the storage portion 22, the control portion 28 takes out
the position signals S0 and S1 specifying the corresponding most
appropriate positions of the disk and the temperatures at the time
of the searches from the storage portion 22 on the basis of the
disk identification information. In this case, it is also possible
to set the most appropriate position of the aberration correction
lens 4 by calculating the target position at the current
temperature by performing the temperature correction in the same
manner as above.
[0115] As has been described, according to the third embodiment, it
is possible to perform more precise recoding and playback because
the temperature compensation for the spherical aberration can be
performed by a simple computation.
[0116] The summary of the third embodiment is set forth as
follows.
[0117] (1) The temperature sensor that detects the temperature of
the optical head is provided.
[0118] (2) The optical head and the control portion that adjusts an
applied voltage to the piezoelectric element according to the
detection result on the optical head by the position detection
portion are provided, and the control portion is configured to
correct the set position of the aberration correction lens on the
basis of the temperature detected by the temperature sensor.
[0119] As in the second embodiment, more than one hall element 13
may be disposed in the drive shaft direction in the third
embodiment, too.
Fourth Embodiment
[0120] FIG. 11 is a view showing a major portion of an optical head
according to a fourth embodiment of the invention. The optical head
includes the hall element 13 as a first magnetic field detection
portion and a hall element 14 as a second magnetic field detection
portion. The rest of the configuration is the same as the first
embodiment.
[0121] The hall element 14 and the hall element 13 comprise hall
elements of the same type. These two hall elements 13 and 14 are
disposed side by side in the moving direction of the aberration
correction lens 4 while being spaced apart from each other.
[0122] An example of the position signals outputted from the
respective hall elements 13 and 14 is shown in FIG. 12. In the
drawing, the position signal from the hall element 13 is indicated
by a solid line and the one from the hall element 14 is indicated
by a dotted line. As is shown in the drawing, by providing plural
hall elements 13 and 14, it is possible to cover the entire movable
range of the aberration correction lens 4. The spatial resolution
can be therefore enhanced.
[0123] Although detailed operations by this configuration are
omitted herein, when an optical layer having two recording layers
is used, it is possible to set a center region generally having a
satisfactory linearity in the position signal from each hall
element to correspond to the position of each recording layer. For
example, the position control of the aberration correction lens 4
is performed according to the position signal from the hall element
13 when an access is made to the first recording layer L0, and
according to the position signal from the hall element 14 when an
access is made to the second recording layer L1.
[0124] The summary of the fourth embodiment can be described that
more than magnetic field detection portion is provided and aligned
in the drive shaft direction.
[0125] As in the second embodiment, it may be configured in such a
manner that more than one hall element 13 is provided in the drive
shaft direction in the fourth embodiment, too.
Fifth Embodiment
[0126] FIG. 13 is view schematically showing a major portion of an
optical disk device according to a fifth embodiment of the
invention. Besides the hall element 13 mounted on the aberration
correction unit 101, a hall element 17 as an example of a
correction magnetic field detection portion is mounted on an
optical head 202 in the optical disk device. The rest of the
configuration is almost the same as the second embodiment.
[0127] The hall element 17 and the hall element 13 comprise hall
elements of the same type. The hall element 17 is disposed so that
the orientation of the magnetic flux sensitivity becomes almost the
same as that of the hall element 13. An output from the hall
element 17 is inputted into a control portion 30 as a reference
signal 29.
[0128] When a hall element is subjected to influences of an
external magnetic field, generally, its characteristic changes with
temperatures or the like. The influences of an external magnetic
field or the like to which is subjected the hall element 17 that is
irrespective of the position signal are equivalent to the
influences of an external magnetic field or the like to which is
subjected the hall element 13 that outputs the position signal.
Hence, by providing the hall element 17, it is possible to detect
these influences alone at the hall element 17. Also, by performing
a computation to correct the position signal from the hall element
13 using the reference signal 29 from the hall element 17 in the
control portion 30, it is possible to reduce the influences of an
external magnetic field, the temperature characteristic, etc. In
addition, by providing the hall element 17, it is possible to
reduce the influences even in a transitional state in which the
temperature changes abruptly like at the moment immediately after
the power supply is switched ON.
[0129] Another configuration may be adopted instead of the
configuration in which the hall element 17 is mounted on the
optical head 202. For example, by incorporating the hall element 17
in an output correction circuit of the hall element 13 using an
operational amplifier, it is possible to achieve the configuration
in which an automatically corrected position signal is inputted
into the control portion 30. In this case, it is necessary to
incorporate the hall element 17 so that the influences of an
external magnetic field and the temperature characteristic to an
output will have the polarity opposite to the polarity of the
influences to the hall element 13. When configured in this manner,
the need to correct the position signal in the control portion 30
can be eliminated.
[0130] The summary of the fifth embodiment is set forth as
follows.
[0131] (1) In this embodiment, the correction magnetic field
detection portion is provided at a position unsusceptible to the
magnetic field developed by the magnetic field generation portion
in such a manner that the direction of the magnetic field
sensitivity is aligned with that of the magnetic field detection
portion.
[0132] (2) The optical head and the control portion that adjusts an
applied voltage to the piezoelectric element according to the
detection result on the optical head by the position detection
portion are provided, and the control portion is configured to
correct the set position of the aberration correction lens
according to the detection result on the optical head by the
correction magnetic field detection portion.
[0133] As in the second embodiment, more than one hall element 13
can be disposed in the drive shaft direction in the fifth
embodiment, too.
Sixth Embodiment
[0134] FIG. 14 is a view schematically showing a major portion of
an optical disk device according to a sixth embodiment of the
invention. An optical head 203 is different from the counterpart in
the fifth embodiment in that a magnet 18 as an example of a
correction magnetic field generation portion is provided. The rest,
including a reference signal 31 and a control portion 32, are the
same as the counterparts in the fifth embodiment.
[0135] The magnet 18 is different from the magnet 12 in the
aberration correction unit 101 provided to the movable lens holder
10 in that it is fixed to the optical head 203. Both of the magnets
12 and 18, however, are common in that they are made of the same
material.
[0136] In the sixth embodiment, in addition to a reduction of the
influences of an external magnetic field, the temperature
characteristic, a transitional response, etc., a correction of the
operating characteristic of the hall element 13 including the
temperature characteristic of the magnet 12 under the actual
magnetic field is enabled. In the control portion 32, it is thus
possible to correct an output signal from the hall element 13 in
the aberration correction unit 101 in response to a change of an
output of the hall element 17. For example, because the influences
of a gain fluctuation or the like caused by a temperature change
appear more apparently than in the fifth embodiment, more precise
correction is enabled.
[0137] For example, let V11 and V12 be outputs of the hall element
13 and 17, respectively, at the position P0 corresponding to the
recording layer L0 at a given reference temperature T1. Let V21 and
V22 be outputs of the hall element 13 and 17, respectively, at the
position P0 corresponding to the recording layer L0 at a given
temperature T2. Then, because a rate of the gain fluctuation is the
same for both the hall elements 13 and 17, we get
V21/V11=V22/V12.
[0138] Initially, the outputs of the hall elements 13 and 17 at the
position P0 corresponding to the recording layer L0 at the
temperature T1 are stored as V11 and V12, respectively. When the
movable portion 100 moves due to a disturbance or the like after
the temperature reaches T2, the hall element 13 is subjected to the
influences of a change of the magnetic field caused by a
displacement of the magnet 12 and the influences of a temperature
change. Meanwhile, because the hall element 17 is subjected to the
influences of a temperature change alone, it is possible to observe
V22.
[0139] Suppose the hall element 13 is subjected to the influences
of the temperature alone at the position P0 corresponding to the
recording layer L0, then the output V21 can be predicted as
V21=V11.times.(V22/V12). Hence, by adjusting the position of the
aberration correction lens 4 so that an output of the hall element
13 reaches V21, the influences of temperature can be reduced. It is
thus possible move the aberration correction lens 4 to the position
P0 corresponding to the recording layer L0 more precisely.
[0140] The configuration of the sixth embodiment enables a
correction of the position signal including the influences of the
temperature characteristic of the magnet 12. In other words, even
when the characteristic of the magnet 12 varies with temperatures,
it is possible to reduce the influences. In addition, because the
magnet 18 provides the hall element 17 with the magnetic field
intensity almost equal to average magnetic field intensity close to
the intensity of the magnetic field provided to the hall element 13
from the magnet 12, it is possible to correct a change of the
sensitivity characteristic of the hall element 13 with temperatures
at this magnetic field intensity.
[0141] The summary of the sixth embodiment can be described that
the correction magnetic field generation portion is provided
adjacently to the correction magnetic field detection portion.
[0142] As in the second embodiment, more than one hall element 13
can be disposed in the drive shaft direction in the sixth
embodiment, too.
Seventh Embodiment
[0143] FIG. 15 and FIG. 16 are views schematically showing a major
portion of an optical head according to a seventh embodiment of the
invention.
[0144] The optical disk 1, the laser light source 3, the objective
lens 5, the aberration correction lens 4, the drive shaft 7, the
frictional holding body 8, the piezoelectric element 6, the magnet
12, and the hall element 13 are configured in the same manner as
the respective counterparts in the first embodiment. A lens holder
50, a guiding groove 50a, an aberration correction base 51, a
bottom portion 51a, and second supporting portions 51c function in
the same manner as the respective counterparts in the first
embodiment.
[0145] An auxiliary guiding shaft 52 is made of a soft magnetic
body. The lens holder 50, the aberration correction lens 4, the
magnet 12, and the frictional holding body 8 together constitute a
movable portion 104.
[0146] The magnet 12 is disposed directly below the guiding groove
50a in FIG. 16. In other words, a direction heading from the magnet
12 to the auxiliary guiding shaft 52 is a direction almost
perpendicular to a direction of the drive shaft 7. The magnet 12
and the auxiliary guiding shaft 52 are disposed on the same
circumference about the drive shaft 7 to almost coincide with each
other.
[0147] Because the auxiliary guiding shaft 52 is made of a soft
magnetic body, it is attracted toward the magnet 12. Hence, as is
shown in FIG. 16, the lens holder 50 is subjected to an upward
force F in the drawing. The movable portion 104 thus starts to
rotate in the counterclockwise direction of FIG. 16 about the drive
shaft 7, which brings the guiding groove 50a and the auxiliary
guiding shaft 52 into contact with each other.
[0148] Generally, when the aberration correction lens 4 moves
abruptly, a light spot may be displaced on the optical disk 1, the
servo becomes instable, and so forth. It is therefore preferable to
reduce the backlash associated with a clearance of the guiding
groove 50a and the guiding shaft 52 to the least possible level.
However, when the clearance is made too small, double friction
restriction, that is, friction restriction by the guiding groove
50a and the guiding shaft 52 and friction restriction by the drive
shaft 7 and the frictional holding body 8, may possibly occur. This
takes place when the guiding groove 50a has a finite width and the
drive shaft 7 and the auxiliary guiding shaft 52 have a skew
positional relation. No mechanism is able to reduce an error of
such a positional relation to 0, and when the clearance between the
guiding groove 50a and the auxiliary guiding shaft 52 is made too
small, it may cause a malfunction as the aberration correction
unit. Normally, a clearance to some extent is secured and a
pressure bar spring or the like is used to remove the backlash.
[0149] In the seventh embodiment, the backlash is prevented using a
force F induced by the magnet 12 that is used to detect the
position of the aberration correction lens 4. Hence, in this
embodiment, it is possible to remove an instability, such as the
backlash associated with a clearance between the auxiliary guiding
shaft 52 and the guiding groove 50a using the attraction force F of
the magnet 12 alone without having to add any new component. It
should be noted that because the hall element 13 is present on the
opposite side of the auxiliary guiding shaft 52, the
attraction-induced change of the magnetic field gives little
influences to the position signal.
[0150] The configuration of the seventh embodiment can be more
compact than the configuration of the first embodiment.
[0151] The summary of the seventh embodiment is set forth as
follows.
[0152] (1) The auxiliary guiding shaft made of a soft magnetic
material and disposed parallel to the drive shaft is provided, and
the magnetic field generation portion is disposed at a position at
which a direction heading from the magnetic field generation
portion to the auxiliary guiding shaft becomes perpendicular to the
drive shaft.
[0153] As in the second embodiment, more than one hall element 13
can be disposed in the drive shaft direction in the seventh
embodiment, too.
Eighth Embodiment
[0154] FIG. 17 and FIG. 18 are views schematically showing a major
portion of an optical head according to an eighth embodiment of the
invention. In the eighth embodiment, an entire aberration
correction unit 105, including the movable portion 104, is of the
same configuration as described in the seventh embodiment.
[0155] In the optical head of this embodiment, the laser light
source 3 is disposed on the opposite side of the seventh embodiment
with respect to the aberration correction lens 4, and a mirror 61
is disposed in a space between the aberration correction lens 4 and
the objective lens 5. The mirror 61 is positioned at a side portion
of the piezoelectric element 6. In this optical head, the mirror 61
is disposed to come in between the drive shaft 7 and the auxiliary
guiding shaft. When configured in this manner, a dead space on the
side of the mirror 61 can be utilized effectively, which can
contribute to a reduction of the optical head in size.
[0156] As a general rule, the piezoelectric element 6 is placed on
an extension of the drive shaft 7, and the piezoelectric element 6
can be accommodated just in the side portion of the mirror 61.
Also, the magnet 12 that moves together with the movable portion
104 is also accommodated in the mirror side portion, which can
contribute to improvements of a space factor.
[0157] The summary of the eighth embodiment is set forth as
follows.
[0158] (1) The drive shaft is disposed parallel to the optical
disk, and the auxiliary guiding shaft disposed parallel to the
drive shaft and a mirror that deflects a flux of light from the
laser light source to a direction of the normal to the optical disk
are provided. The mirror is disposed in a space between the
aberration correction lens and the objective lens, and is also
disposed in a space between the drive shaft and the auxiliary
guiding shaft.
[0159] As in the second embodiment, more than one hall element 13
can be disposed in the drive shaft direction in the eighth
embodiment, too.
Ninth Embodiment
[0160] In the respective embodiments described above, the magnet 12
is a magnet comprising two wedge-shaped region as are shown in FIG.
4 and FIG. 5. The magnet 12, however, is not limited to those shown
in FIG. 4 and FIG. 5.
[0161] A relation of the magnet and the hall element when magnets
of other embodiments are used will be described with reference to
FIG. 19A through FIG. 19C. FIG. 19A shows a major portion when a
simple bar-shaped magnet 12 is used. As in the respective
embodiments described above, the magnet 12 and the aberration
correction lens 4 move mechanically as one unit via the aberration
correction base 11, the lens holder 10 and the like. The magnet 12
and the hall element 13 oppose each other, and a quantity of
magnetic flux added to the hall element 13 varies with the relative
position. The hall element 13 generates an output signal
corresponding to the position of the aberration correction lens 4.
This configuration can save the cost of components because a simple
magnet can be used.
[0162] FIG. 19B shows a magnet comprising two-divided portions in
the shape of wedge, and because it is the same as the magnet shown
in FIG. 4 and FIG. 5, descriptions are omitted herein. The magnet
12 may be formed by laminating two magnets or polarizing one magnet
in two divided portions. When configured in this manner, the
sensitivity becomes high and the linearity of the conversion
characteristic of the placed position of the aberration correction
lens 4 to a position signal becomes satisfactorily. Aberration can
be therefore corrected more precisely.
[0163] FIG. 19C shows an example in the case of using a relatively
short bar magnet 12. The bar magnet 12 is disposed in a space
between two hall elements 13a and 13b disposed oppositely, and is
configured to be movable between the hall elements 13a and 13b.
When configured in this manner, because a simple bar magnet is used
as the magnet 12, a volume occupied by the magnet 12 can be
reduced. Also, because an interval from the magnet 12 to the hall
elements 13a and 13b can be set larger, a risk of mutual contact or
collision can be lessened to the least. In addition, the need of an
accurate adjustment of a gap between the magnet 12 and the hall
elements 13a and 13b is eliminated. Moreover, by detecting a
difference between outputs from the two hall elements 13a and 13b,
it is possible to cancel out the noises or cancel out the
temperature characteristic.
[0164] In the embodiments shown in FIG. 19A and FIG. 19C, the same
idea as the one adopted to the magnet 18 and the hall element 17 in
FIG. 14 can be applied. In other words, an output signal from the
hall element in FIG. 19A and FIG. 19C can be corrected by obtaining
a signal equivalent to a reference position signal by adding a
magnetic flux of the reference magnet to the reference hall
element. In the case of FIG. 19C, two reference magnets and two
hall elements may be provided.
[0165] In the respective embodiments described above, an example
using the hall element as the magnetic field detection portion has
been described. The invention, however, is not limited to this
example. For example, an MR element or the like can be used as
well.
[0166] Also, in the respective embodiments described above, an
example in a case where the hall element used as the magnetic field
detection portion is mounted on the aberration correction base has
been described. The invention, however, is not limited to this
example. In short, any configuration is possible as long as a hall
element is provided in a portion that does not move relatively with
respect to the aberration correction base. For example, the hall
element may be mounted on the optical head at its own base.
[0167] Also, in the respective embodiments described above, it is
configured to use the aberration correction base. The invention,
however, is not limited to this configuration. For example, it may
be formed as a part of the structure of the optical head. Even when
configured in this manner, the invention can be achieved
mechanically without any difference. In short, any structural body
is available as long as it functions as the lens holder, the
frictional holding body, and the aberration correction base.
[0168] There is no technical problem in fabricating an optical disk
device incorporating the optical head of the fourth, seventh, or
eighth embodiment. For example, such an optical disk device can be
fabricated using the same configuration as the second and third
embodiments. In these cases, the resulting device can also achieve
the effect of the invention effectively.
INDUSTRIAL APPLICABILITY
[0169] The invention can be used as an optical head that irradiates
a flux of light from the laser light source onto an optical disk
through the objective lens.
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